Light sources vary across a large range of brightness. The human eye can adapt to a wide range of average brightness by adjusting the size of the iris and by varying the chemical response in the retina, but human eyes have difficulty adjusting to a wide range of brightness within a single scene. Bright sources of light within a scene are often perceived as glare. Glare-shielding sunglasses have been developed to reduce glare. In general, glare-shielding sunglasses use an image sensor to detect a glare source, and then certain of the addressable light valves in the lenses of the sunglasses are caused to decrease the amount of light being transmitted according to the location of the glare source. To fit the necessary electronics and their associated power supply into a product as lightweight as a pair of sunglasses, the electronics must be designed to consume as little power as possible.
Existing image sensors detect light falling on each pixel of a light receiving surface. FIG. 1 is a schematic of such a sensor. Such image sensors usually have circuitry (“sensor circuits”) located adjacent to the light receiving surface, which accept charges from each pixel and convert those charges into signals that can be transmitted. Signals from each pixel may be in the form of an analog voltage, charge, or current, and the sensor circuits may amplify, digitize, and/or convert the signals to one of the other forms. Next, the signals from the sensor circuits are sent to processing circuits, which usually are not packaged with the light receiving surface, but instead are located remotely from the light receiving surface. The processing circuits determine whether glare is present on the light receiving surface, and if glare is present, how to reduce the glare.
For example, charge-coupled devices (CCDs) typically move an electric charge stored at each pixel to sensor circuits at the edge of the light receiving surface, where the charge is converted to an analog voltage, which is then transmitted to the processing circuits. Complementary metal-oxide-semiconductor (CMOS) image sensors transmit an analog voltage or current to the sensor circuits, which can strengthen the signal through amplification, process the signal to suppress noise, and convert the format of the signal so that it conforms to specific electronic standards, such as the NTSC standard for television signals. Some existing sensor circuits include analog-to-digital (A/D) converters, so that only digital signals are sent to the processing circuits. However, the signal sent from the pixel to the sensor circuits is analog because to form a picture, numerous shades are necessary. Analog signals consume a large amount of power, and it would be helpful to reduce the power consumption needed to detect glare.